Display devices are classified, according to a display manner, into self-luminescent devices such as cathode ray tube (CRTs), plasma display panels (PDPs), light emitting diodes (LEDs), organic light emitting diodes (OLEDs), field emission displays (FEDs), and the like, and passive type devices such as liquid crystal displays (LCDs), digital lighting processing (DLP), and the like, which block and reflect light emitted from a separate light source. Further, according to a drive manner, the display devices are classified into active matrix type devices such as TFT-LCD and AMOLED, and passive matrix type devices such as PDPs and PMOLEDs.
Further, the self-luminescent devices are generally classified, according to a light emission manner, into photoluminescent devices, such as PDPs, which emit light through excitation by photons, electroluminescent devices, such as OLEDs and LEDs, which emit light through excitation by electrons, and cathodoluminescent devices, such as CRTs and FEDs, which emit light through excitation by accelerated electrons.
The studies on OLEDs were started in the early 1980s by C. W. Tang of Eastman Kodak Co. Ltd., in the U.S. In 1987, a thin film OLED exhibiting brightness of 1000 cd/m2 at an operation voltage of 10V or less was manufactured by introducing a stack structure of a hole transport layer (HTL) and an electron transporting layer (ETL) and doping a fluorescent dopant into an emitting layer (EML). Since then, OLEDs have been rapidly developed.
Development of low molecular weight material OLEDs promoted by Kodak resulted in commercialization through continuous studies by Pioneer Co. Ltd., Idemistu Co., Ltd., and the like.
Further, PPV issued by Friend's Group at Cambridge University in England triggered studies on polymer materials for OLEDs.
A solar cell is a semiconductor device which directly converts sunlight into electricity based on photovoltaic effects by which electrons are generated by illuminating light to a p-n junction semiconductor diode.
Such solar cells are generally classified into bulk type solar cells based on a crystalline Si wafer, and thin film type solar cell, such as non-crystalline silicon, CdTe, CIGS, dye sensitized solar cells (DSSC), organic solar cells, according to the kind of material capable of absorbing sunlight.
Since an OLED has a structure similar to a thin film type solar cell, efforts have been made to realize generation of electricity and display of information at the same time using the OLED and the thin film solar cell.
For each of the OLED and the solar cell, the materials of layers are selected in consideration of the energy level between the layers such as a pair of electrode layers, a light emitting layer, and a photoelectric conversion layer. If the energy level is not taken into consideration, electron mobility is lowered, causing deterioration in emission efficiency of the OLED and in electricity generation efficiency of the solar cell.
Korean Patent Laid-open Publication No. 10-2005-0083243 (Document 1) and Korean Patent Laid-open Publication No. 10-2008-0065120 (Document 2) disclose a solar cell integrated display device which includes a solar cell joined to one side of an OLED.
However, according to these documents, since the OLED has a well-known structure, there is no substantial problem relating to light emission efficiency. However, the solar cell is formed without any consideration of the energy level, causing low efficiency in generation of electricity.
Further, at least one of a pair of electrodes for the light emitting diode is composed of indium tin oxide (ITO) to form a transparent electrode in order to allow light emitted from the light emitting diode to be transferred to a user, and at least one of a pair of electrodes for the solar cell is composed of ITO to form a transparent electrode in order to allow sunlight to be transferred to the photoelectric conversion layer.
However, an organic layer under the electrode layer is likely to be damaged by heat generated upon deposition of ITO.
Particularly, in the structure wherein the OLED and the organic solar cell share a certain electrode at a combined side thereof and sunlight is transferred to the photoelectric conversion layer of the organic solar cell through the organic light emitting diode, it is necessary to use a shared electrode layer as a transparent electrode, and when the transparent electrode is composed of ITO, the organic layer under the electrode is damaged.